This invention relates to predistortion circuits for radio frequency power amplifiers and more particularly to predistortion circuits utilizing ferrimagnetic materials.
Radio frequency (RF) power amplifiers for microwave communications systems are typically either traveling wave tube amplifiers (TWTA) or solid state power amplifiers (SSPA). Both amplifier types undesirably exhibit a phase shift to signal passage therethrough which changes as a function of input power. This change in phase shift is particularly undesirable in communications systems including satellite communications systems in that the change in phase shift causes signal distortion. Prior art attempts to reduce the change in phase shift by reducing input drive power and thus derating the power amplifier output power and by feedforward methods using an additional power amplifier for enhancing signal linearity are inefficient. Derating amplifier output power, given the usual requirement for a given operating output power level, is tantamount to using a larger TWTA or adding more SSPA power output stages, either of which is costly in hardware weight and volume. These methods require considerably more d.c. power and dissipation. For a satellite, this also requires more solar cells for sustaining the larger d.c. power load.
A device called a magnetostatic surface wave signal-to-noise enhancer has been reported in an article by the same name by S. N. Stitzer, J. D. Adam et al. in 1980 IEEE MTT-S International Microwave Symposium Digest, Washington, DC, May 28-30, 1980, (NY, USA:IEEE 1980), pp. 238-240 and in an article entitled "A Magnetostatic Wave Signal-to-Noise Enhancer" by J. D. Adam and S. N. Stitzer in Applied Physics Letters, Vol. 36(6), Mar. 15, 1980, pp. 485-487. The signal-to-noise enhancer is a passive two port device which performs the function opposite that of a frequency selective microwave ferrite power limiter. In the enhancer strong (high powered) incoming signals are attenuated 20 to 30 dB less than are weak signals.
Such a device also has the normally undesirable property of phase shifting the signal passing therethrough by an amount which is a function of input power. This undesirable property is illustrated in a final report by the aforementioned Adams and Stitzer entitled "Magnetostatic Surface Wave Signal-to-Noise Enhancer" dated October 1980 and available from Defense Technical Information Center, Cameron Station, Alexandria, VA 22314, as Manual No. ADB054545. In FIG. 30B, page 39 thereof from which FIG. 6 of the instant application is an approximation, a series of relative phases at different input power levels and different frequencies are illustrated. From a review of that FIG. 30B and accompanying text it is understood that expected operation of the enhancer is over a wide bandwidth about a center frequency at various input power levels. At frequencies lower than the center frequency, there is a phase shift in one direction which changes as a function of input power and at frequencies higher than the center frequency there is a phase shift which changes in the opposite direction as a function of input power. Such phase shift changes, while not desired, are tolerated in the enhancer operation. The present invention, rather than merely tolerating the change in phase shift with input power, utilizes such change in phase shift as a function of input power for purposes of linearizing the system.